1. Technical Field
The present disclosure relates to a method for transferring a graphene layer onto a substrate.
The disclosure refers, in particular, but not exclusively, to a method for transferring a graphene layer from a donor substrate to a different substrate, of the type comprising a rigid or a flexible material, and the following description is made with reference to this field of application just for explanation convenience.
2. Description of the Related Art
In recent years, many studies have been done on the production and transferring of graphene, which is a material comprising a single layer of carbon atoms placed in an hexagonal crystal lattice and having excellent electrical and optical properties. It is well known, in fact, that graphene is a zero gap semiconductor material having high charges mobility (10,000 cm2/Vs), high conductivity, high transmittance (˜98%) and excellent mechanical properties (high tensile stress equal to 130 GPa). Graphene has a honeycomb geometry such that the electrons of the carbon atoms are forced to move along the plane defined by the graphene sheet following hexagonal paths. Consequently the electrons behave as no mass particles having a speed light like. Due to these particular properties, graphene seems to be qualified to be used in nano-electronic applications, sensors applications and sustainable energy applications, such as:
photovoltaics;
Touch screens, organic light emitting diodes (OLEDs), liquid crystal displays;                sensors;        components of an integrated circuit (such as bipolar transistors, FETs . . . );        large area devices; and        flexible integrated circuits, to name few.        
Geim and Novoselov, Manchester University, won the Nobel prize for having extracted a graphene mono-atomic layer from a graphite bulk material.
In fact, a graphene sheet, having hybridized sp2 carbon atoms, can be considered as the base structure of other graphitic materials, like fullerene (OD), carbon nanotubes (1D), graphite (3D). Graphite, in particular, has a crystal lattice comprising stacked layers linked together by Van der Waals-like inter-layer bonds having energy equal to 2 eV/nm2. Consequently, graphite is easily exfoliable along a direction parallel to the crystal plane exercising forces around 300 nN/mm2.
Different methods have been proposed for graphene production and in particular:                a mechanical exfoliation of graphite, described in “Electric Field Effect in Atomically Thin Carbon Films”, K. S. Novoselov et al., Science, 306, 666-669 (2004); and in “Graphene Transistors Fabricated via Transfer-Printing In Device Active-Areas on Large Wafer”, Xiaogan Liang, Zengli Fu, and Stephen Y. Chou, Nanoletters (2007);        a graphitization of silicon carbide, described in “Towards Wafer-Size Graphene Layers by Atmospheric Pressure Graphitization of Silicon Carbide”, Emtsev, K. V. et al., Nature Mater. 8, 203-207 (2009);        a chemical vapor deposition (CVD) for growing metallic substrates, described in “Graphene Segregated on Ni Surfaces and Transferred to Insulators”, Qingkai Yu, Jie Lian, Applied Physics Letters 93, 113103 (2008); and in “Graphene Films with Large Domain Size by a Two-Step Chemical Vapor Deposition Process”, Xuesong Li, et al. Nano Lett. (2010);        an exfoliation in liquids, described in “Synthesis of Water Soluble Graphene”, Yongchao Si and Edward T. Samulski, Nano Lett. (2008); and in “Chemical Methods for the Production of Graphenes”, Sungjin Parkl and Rodney S. Ru, Nature Nanotechnology (2009); and        carbon nanotubes “unzipping”, described in “Mechanism of Carbon Nanotubes Unzipping into Graphene Ribbons”, Norma L. Rangel, Juan C. Sotelo, and Jorge M. Seminario, THE JOURNAL OF CHEMICAL PHYSICS (2009).        
More in detail, the mechanical graphite exfoliation comprises applying onto the crystal surface of highly oriented graphite (HOPG) a force, usually using an adhesive tape, for peeling and deploying the crystal layers in order to obtain an isolated single layer. This technique is very simple and accessible for isolating graphene flakes of some square microns, and it is used especially in the University research projects. Moreover, the graphene samples obtained have optimal electric and mechanical properties. Nevertheless, even if this method is very simple and low cost, it cannot be used for industrial production, due to the reduced dimensions of the graphene sheets so produced.
In case of the graphitization of silicon carbide (SiC), the graphene is obtained by a high temperature annealing of a silicon carbide crystal which is exfoliated along the direction of the Si-rich (0001) plane or the C-rich (0001) plane. This technique has very high costs and is limited by the diameter of the silicon carbide wafer. In addition, the produced graphene has many defects due to the silicon carbide superficial morphology.
Furthermore, the technique using chemical vapor deposition (CVD) involves poly-crystal substrates of different transition metals, such as nickel, copper, iridium, platinum, which act as catalyst for the growth of the graphene. As an example, a nickel film is exposed to a gas flow comprising hydrocarbons and hydrogen at a temperature equal to 900°/1000° C. At these temperatures, carbon forms a solid solution with nickel, so that, cooling the substrate at high speed, the carbon forms one or more graphene layers in the solution. As an example, using copper, the substrate is exposed to a methane/hydrogen flux at 1000° C., forming CxHy. The whole surface of such a substrate is covered by graphene nuclei formed in specific conditions of pressure, density and temperature. The advantage of this technique is that has a low cost and provides good quality graphene monolayer films, especially using copper.
On the other hand, the technique providing a graphite exfoliation in liquid phase within an organic solvent allows to obtain colloids of graphene sheets. The exfoliation is promoted by sonication, particularly for a solvent having a superficial energy equal to the graphene energy, such an exfoliation being due to the interaction between the solvent and several graphene layers. Good solvents for exfoliation are: N-methylpyrrolidone (NMP), N-dimethylacetamide (DMA), N-dimethylformamide (DMF), G-butyrolactone (GBL), and others of the same type. Even if a large number of graphene monolayer films are so produced, the total weight being up to 50%, the solvents which have to be used are too expensive and dangerous.
At the end, a carbon nanotubes “unzipping” can be realized in different manner:                by a chemical etch of nanotubes with sulfuric acid and potassium permanganate as oxidant agent;        by a chemical etch of nanotubes partially immerged in a polymeric material in presence of Argon plasma;        by introducing alkali metal atoms among the concentric cylinders of carbon nanotubes and obtaining the graphene sheets with “lift-off” techniques.        
Even if all the above described chemical techniques are advantageous for the scalability and flexibility, they do not guarantee a control of the number of dispersed graphite layers. Consequently, the isolation of a single graphene monolayer should become possible only using complicated separation techniques.
In particular, for the graphene production techniques based on the Chemical Vapor Deposition (CVD) growth on metallic substrates or based on exfoliation, the problem of transferring the graphene monolayer on an insulating substrate has recently been studied.
There are also four known, well-used transferring techniques.
1) Transferring using a thermal release tape, as described, for example, in the US patent application published on Mar. 3, 2011 under N. US 2011/0048625 in the name of Caldwell et al. This technique in particular comprises the following phases, described with reference to FIGS. 1A-1E:                laminating an adhesive thermal release tape 1 on a metal layer 2, such as nickel (Ni) or copper (Cu), formed on a silicon oxide layer 3 formed on a silicon substrate 4, the metal layer 2 having on its surface, opposite to the silicon oxide layer 3, a graphene layer obtained by a chemical vapor deposition (CVD), or CVD graphene layer 5, as shown in FIG. 1A;        detaching the metal layer 2 with the CVD graphene layer 5 and the adhesive thermal release tape 1 from the structure comprising the silicon substrate 4 and the silicon oxide layer 3, as shown in FIG. 1B;        chemically etching the metal layer 2 to temporary transfer the CVD graphene layer 5 on the adhesive thermal release tape 1, as shown in FIG. 1C;        laminating the adhesive thermal release tape 1 on a generic substrate 6, as shown in FIG. 1D; and        thermally releasing from the adhesive thermal release tape 1, thus transferring the CVD graphene layer 5 on the substrate 6, as shown in FIG. 1E.        
Even if this technique is advantageous under many aspects, such as high productivity and scalability on a large area, it has the drawback that the glue on the tape is randomically kept on the transferred graphene layer, causing breaks on it.
2) Transferring through a liquid resist, such as poly-methyl methacrylate or PMMA, this technique uses a liquid resist, in particular the PMMA, as a support for temporarily keeping a graphene layer. In particular, the technique comprises the following phases, described with reference to FIGS. 2A-2F:                growing a graphene layer 15 on a metal layer 12, such as nickel (Ni) or copper (Cu), formed on a silicon oxide layer 13 formed on a silicon substrate 14, as shown in FIG. 2A;        depositing a PMMA layer 11 on the graphene layer 15 through a “spin coating” or “casting” technique and, then, thermally stabilizing the graphene layer 15 and the PMMA layer 11 on it, as shown in FIG. 2B;        optionally wet etching the silicon oxide layer 13, as shown in FIG. 2C;        wet etching the metal layer 12, releasing the graphene layer 15 from the silicon substrate 14 and temporary transferring the graphene layer 15 on the PMMA layer 11, as shown in FIG. 2D;        transferring the PMMA layer 11 on a generic substrate 16, as shown in FIG. 2E;        wet etch removal of the PMMA layer 11, thus transferring the graphene layer 15 on the generic substrate 16, as shown in FIG. 2F.        
This technique has the drawback that it is not scalable on a large area and it is not used for mass production.
3) Transferring through polydimethylsiloxane or PDMS which is considered as a direct transfer. The technique is similar to those above described, apart that a PDMS layer is used both as support layer to transfer the graphene layer and also as a final substrate for it.
4) Transferring through a so called “Self assembly”, which is a technique for generating and transferring highly ordered organic films with controlled thickness, so obtaining graphene flakes. This technique comprises:                dispensing, on the sub-phase surface inside an equipment pan, a solution of an organic substance, in particular graphene, to be deposited;        waiting for the complete evaporation of the solvent;        compressing the graphene film through barriers which reduce the area available for the molecules at the surface of the sub-phase;        transferring the graphene film by immerging and emerging perpendicularly the substrate from air to the sub-phase, according to the so called “Langmuir-Blodgett” deposition, or in a parallel manner by contacting the substrate surface with the film surface, according to the so called “Langmuir-Schaefer” deposition.        
This technique has the drawback that the graphene productivity is very limited and involves a transferring on very small areas.